1. Field of the Invention
The present invention relates to a photography technique using an automatic image correction function.
2. Description of the Related Art
An imaging apparatus is conventionally equipped with an exposure control unit for determining exposure parameters to capture an image with suitable brightness when the image is captured. To obtain an appropriate exposure, the exposure control unit determines a combination of the exposure parameters, such as a shutter speed (or a charge accumulation time), an aperture value, and an International Organization for Standardization (ISO) sensitivity, based on light metering results before capturing an image. However, for example, when an image is captured against the sun or when a highly reflective object is included in an image, a user cannot always find the captured image satisfactory. Further, when an interchangeable-lens system is used, decrease in marginal illumination is caused depending on characteristics of an attached lens, and corners of the captured image is darkened.
Japanese Patent Application Laid-Open No. 2005-223511 discusses a technique for obtaining image data with appropriate brightness. According to the technique, an image is captured with exposure parameters determined by light metering results and image correction processing is then executed on the image data.
Since an ISO sensitivity of film cameras is determined by a film to be used, a combination of the shutter speed and the aperture value is used as the exposure parameters. In contrast, in the case of digital cameras, the ISO sensitivity is also changeable. Recently, an automatic ISO sensitivity setting mode, in which the sensitivity is automatically set, has been installed in digital cameras in addition to a manual sensitivity setting mode set by a user. Therefore, for digital cameras, in addition to the shutter speed and the aperture value, the ISO sensitivity can also be set as an exposure parameter. For example, Japanese Patent Application Laid-Open No. 2005-292740 discusses a digital camera which can prevent camera shake by automatically increasing the ISO sensitivity in a low luminance environment and controlling the shutter speed not to decrease.
However, when an image is captured with a high ISO sensitivity, noise in the image tends to increase. Thus, to prevent a signal-to-noise ratio (SNR) of an image from falling below a predetermined level, an upper limit in a selectable range of an ISO sensitivity is generally determined in an automatic ISO sensitivity setting mode, compared with the upper limit in a selectable range of the ISO sensitivity in a manual ISO sensitivity setting mode.
For example, while the ISO sensitivity can be set in the range of 100 to 1600 in the manual ISO sensitivity setting mode by the user, the ISO sensitivity can be set in the range of 100 to 800 in the automatic ISO sensitivity setting mode. In this case, it is assumed that a captured image satisfies a predetermined SNR level with the ISO sensitivity 800. However, if the above image correction processing amplifies a signal to increase luminance of image data, when the image correction processing is executed on the captured image, noise is accordingly amplified further, and the SNR may be deteriorated.